Magnetic random access memory (MRAM) is a new technology that will likely provide superior performance over existing flash memory technology and may even supplant disk drives in certain low power applications requiring a compact non-volatile memory device. In MRAM, bits are represented by the magnetic configuration of a small volume of ferromagnetic material and its magnetic state is measured via a magnetoresistive (MR) effect during read-back. The MRAM typically includes a two-dimensional array of cells, with each cell containing one MR element that can store one bit.
Most common MRAM designs employ MR elements that are based on either giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR), where these elements are patterned thin film multilayer structures that include at least one pinned ferromagnetic layer (PL) and one free ferromagnetic layer (FL). These elements are designed so that the FL has a bistable magnetic configuration that is preferentially aligned along one out of a possible two anti-parallel directions. Hence, the binary data are stored as the orientation of the FL magnetization. The PL magnetization is aligned along only one of the two possible FL directions, and its role is to provide a fixed reference so that the magnetoresistance reveals the FL orientation with respect to the PL.
There are numerous ways to implement a device based on this concept. However, the leading method to write bits is a so-called “half-select” process in which the magnetic fields generated by two roughly orthogonal current carrying wires orient the free layer into its intended configuration. All publicly disclosed MRAM prototypes to date have used this method. The amplitudes of the currents flowing through the two wires, referred to as the word and bit lines, are chosen so that the corresponding fields reverse a FL only where the two wires intersect. Therefore, this method can select any individual cell within the array with a minimum of electrical wires and without unintentionally reversing the magnetic state of other cells.
Although MRAM has yet to reach the commercial market, it is likely that at least the first generation of products will employ a half-select write process. Unfortunately, this writing scheme exhibits poor scaling with increasing areal density of cells. MRAM faces the same issue of thermal stability that is confronting hard disk drives. In order to preserve the stability of the bits, the magnetic anisotropy of the FL will necessarily have to increase whenever the dimensions of the MR element decrease. This means that higher density MRAM will require larger magnetic fields in order to write bits, which will require larger currents through the word and bit lines that will consume more power. One way to improve the writing efficiency is to employ a thermally assisted writing scheme that reduces the FL anisotropy by heating the MR element. However, this write process will have to overcome numerous reliability issues related to the large increases in temperature required to make this scheme work. Thermally assisted writing will also face significant challenges in producing competitive writing speeds, as the process is fundamentally limited by the time (˜1 ns) it takes for thermal energy to transfer from the lattice (phonons) to the magnetic system.
Another way to improve the writing efficiency of high density MRAM is to set the FL magnetization using the torque from an effect referred to as spin momentum transfer, as has been proposed in certain patents. Spin transfer is a phenomenon that occurs in current perpendicular to the plane (CPP) GMR devices that have cross-sectional areas on the order to 104 nm2 or less. The strength of the torque is directly proportional to the current density through the CPP pillar. Therefore, for a given current, the current density will increase as the area of the CPP pillar decreases in size, and spin transfer will become more efficient as the density of MRAM increases. However, even a write process based on spin momentum transfer will have to confront the increasingly burdensome requirement of having to reverse the magnetization of a FL having a larger anisotropy. Furthermore, initial predictions indicate that a write process based on spin transfer alone can be slower at reversing the FL magnetization than writing with a conventional magnetic field.
There is a need for a MRAM that can overcome the deficiencies of previous MRAM devices.